1. Field of the Invention
The present invention relates to a displacement measuring device and method, for measuring a displacement of an optical disk, and an optical recording/playback apparatus and method, for recording/reproducing information signals to/from an information recording surface of an optical disk.
This application claims the priority of the Japanese Patent Application No. 2003-137764 filed on May 15, 2003, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
Recently, a recorder/player is used which optically records or reproduces information to or from an optical disk. Normally in the recorder/player, there is provided an optical pickup that writes or reads information signals to or from an information recording surface of the optical disk by focusing laser light on the recording surface. The optical pickup is designed to be movable at least radially of the optical disk and detect and write or read information signals along a recording track of the optical disk being rotated.
The optical disk is formed by injection molding of a plastic. The optical disk is likely to be warped due to a change of environmental conditions such as the temperature and humidity. During molding, a distortion, if any, of the molds used for the injection molding will possibly cause a surface undulation of the disk product. Also, the optical disk will possibly be set out of perpendicular to the optical pickup because of a physical relation with a disk seat in the recorder/player.
If an optical disk with such a warp or undulation or an optical disk set out of perpendicular is rotated, the information recording surface thereof will be off the focal point of an objective lens in the optical pickup or have an angle in relation to the focal plane of the objective lens, resulting in extremely poor characteristics of information signal recording and reproduction.
On this account, it has so far been proposed to measure a displacement of an optical disk, caused by a so-called axial runout developed are typically a contact measuring method in which an electrical micrometer is used to measure a displacement of the information recording surface of an optical disk, and non-contact measuring methods including a capacitance method in which a capacitance is detected and a displacement of the recording surface is measured based on the detected capacitance, and an eddy-current method in which an eddy current is produced and a displacement of the recording surface is measured based on the eddy current. Further, there has recently been proposed an axial-runout measuring device which calculates an irregularity or displacement of the information recording surface of an optical disk by measuring a vertical velocity component of an information recording surface of an optical disk with the use of the laser interference and serially adding together the measured results (cf. the Japanese Published Unexamined Patent Application No. 2000-048384).
In case a displacement of an optical disk is measured by the contact measuring method, however, the electrical micrometer has to be put in touch with the optical disk, which will possibly scratch the information surface of the optical disk.
Also, in case a displacement of an optical disk is measured by the capacitance method included in the non-contact measurement methods, since a capacitance detectable by the method is inversely proportional to a gap between the information recording surface of the optical disk and a capacitance detector, so only a very low voltage can be detected, which cannot assure any stable measurement.
To efficiently produce an eddy current in the eddy-current method, the object to be measured should desirably be metallic. However, each of such objects has to be calibrated, which needs excessive labor and costs.
Further, the above axial-runout measuring method using the laser interference needs a separate interference optical system, which leads to a difficulty in designing a compact measuring device. Also, such an interference optical system has to be calibrated with a high accuracy, which cannot be advantageous in the respect of the costs.